1. Field of the Invention
This invention relates to novel halogenated polybutadiene series elastomers, a method of producing the same and a rubber composition for tire containing this elastomer, and more particularly to a novel polybutadiene series elastomer having a halogen group capable of being subjected to heat-resistant crosslinking, a method of producing the same as well as a rubber composition for tire containing this elastomer and having an excellent heat resistance in high-speed and high-performance pneumatic tires continuously used at high temperature state.
The halogenated polybutadiene series elastomers produced by the method according to the invention can be subjected to heat-resistant crosslinking with an amine series compound as mentioned below. When the resulting crosslinked rubber is used in tire parts, particularly tread, carcass and belt of automobile tires, particularly highspeed running tires for racing, motor sport and the like, troubles such as tire burst and the like due to a great amount of heat generated during the running at high speed can be prevented to ensure the stable running of the tire under heat-resistant state.
2. Related Art Statement
Recently, in the automobile tire, the use limit temperature of the tire tread becomes higher in accordance with the improvement of performances in the automobile, completion of expressway network and improvement of high-speed running performances of the tire, and consequently the heat resistance is more highlighted as a performance required in the tread.
Heretofore, a method of increasing number of monosulfide bonds in the crosslinking and the like have been proposed as a technique for improving the heat resistance, but the improved level is still insufficient. Further, the crosslinking with a peroxide provides an excellent heat resistance, but the scorching property is poor and the crosslinking speed is slow, so that such a crosslinking is not practical.
It is known from old times that when a halogen such as chlorine gas or the like is acted to a polybutadiene series elastomer such as styrene-butadiene copolymer rubber, polybutadiene rubber or the like, the addition reaction of halogen to unsaturated bond is produced. However, the resulting product is poor in the practical value, so that the detailed examination thereof is hardly attempted.
In order to ensure the heat stability of rubber, or in order to lessen the degradation at high temperature as far as possible, it is particularly important that the heat stability through crosslinking is high. In this connection, it is known that Friedel-Crafts reaction is carried out between chlorobutyl rubber or bromobutyl rubber and an aromatic amine such as N,N'-dipehnyl-p-phenylene diamine or the like to provide a crosslinked structure having a high heat resistance (D. C. Edwards, Rubber Chemistry and Technology, vol. 60, page 62, 1987). Such a vulcanization is called as TP curing.
In case of styrene-butadiene copolymer rubber frequently used in the tire tread, when this rubber is reacted with a halogen molecule, a simple addition reaction is merely caused, so that it is impossible to obtain excellent heat resistance even by crosslinking the resulting product with the above aromatic amine.
Further, in order to apply such a TP curing to polybutadiene series elastomers such as cis-1,4-polybutadiene, styrene-butadiene copolymer and the like, it is required to effectively introduce an active halogen group into these elastomers, but such an attempt is not yet successful. In this case, a halogen group capable of bonding at an allylhalogen structure or benzylhalogen structure may be mentioned as the active halogen group. Therefore, in order to introduce such an active halogen structure into the polybutadiene series elastomer, it is first considered that a halogen such as chlorine gas or the like is directly reacted with the elastomer. In this case, however, the introduction of allylhalogen structure is not successful because the following addition reaction is usually caused: ##STR1##